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Old 28-07-2012, 04:16   #31
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Huge swells of the 20 to 40 foot variety will pass the boat without any effect. If you close your eyes or are below they may pass undetected as their physics has the local gravity exactly perpendicular to the sea surface. They are a wonder to sail in. The boat will not change direction or speed as they pass. Common along the California coast in the winter months.

Wind driven or breaking waves of 20 feet are altogether different.
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Old 28-07-2012, 06:40   #32
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Re: Sailing between the Swells..

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Thanks for all the responses. 30 foot swells I would be heaving too, and probably heaving my guts out the cockpit too. I think ocean swell management is rarely talked about but sure makes a difference in passage making. The speed of the swells I am sure is some derivative of the frequency, windstrength and height, but I have yet to figure that out.
Jackdale- if you are approaching the top of a swell, what is the best angle to approach it at? Lets say you are broad reached almost perpendicular to the wind. I have seen a drop off a swell and a near broach, so of course you do not want to be parallel. It is an interesting problem. So much to learn!

When it happened to me, we were overpowered with no immediate solution for that, which made it much worse, because the boat WANTED to go broadside -- and then try to broach. I managed to keep the boat from doing this, which made for quite a wild ride, and the other person on the boat was treating it like a fun roller coaster ride and actually refused to take the actions I directed her to take to help me maintain some control of the boat.

It was ugly. Smaller boat, and very narrow.
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Old 28-07-2012, 07:47   #33
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Re: Sailing between the Swells..

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When it happened to me, we were overpowered with no immediate solution for that, which made it much worse, because the boat WANTED to go broadside -- and then try to broach. I managed to keep the boat from doing this, which made for quite a wild ride, and the other person on the boat was treating it like a fun roller coaster ride and actually refused to take the actions I directed her to take to help me maintain some control of the boat.

It was ugly. Smaller boat, and very narrow.
Speaking of overpowered. Reminds me of the time I had full sail up with Mr. NOAA broadcasting gale warnings. The boat was new to me and I was not sure of the reefing procedure. Things started out fine running east with the winds out of the west. But, thirty or forty miles later the fetch made things interesting. Surfing down the front of the waves at unseen of speeds. White knuckled holding on to the helm. Scared and exhilarating at the same time. Made record time but, lost a real nice fiberglass lapstrake dingy three miles from the destination. Tried to go back for it and almost broached. That ended that attempt. At some point remember the crew facing me while I was at the helm and saying "don't look behind you". I think it was good advice. I now reef early and often.
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Old 28-07-2012, 10:56   #34
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Anyone know how to estimate the speed of a swell?
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Old 28-07-2012, 12:04   #35
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Speed of an individual wave in knots is 3 times the wave period in seconds. A open sea wave that is not greatly affected by the wind with a period between crests of 15 seconds will pass your boat at 45 knots.
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Old 28-07-2012, 18:13   #36
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Re: Sailing between the Swells..

OK, this is going to sound silly, but medicine has always been my gig, not physics. How can a wave travel faster than the wind? Our swells are often 10 to 12 seconds apart, wind waves shorter, but rarely does the wind get above 30 knots. How come the swells are all faster than the wind?
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Old 28-07-2012, 19:29   #37
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Re: Sailing between the Swells..

I'm sure there's a few surfers who'd like to find those 45kt waves...
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Old 28-07-2012, 19:35   #38
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Re: Sailing between the Swells..

Oceanography: waves
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Old 28-07-2012, 19:41   #39
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Re: Sailing between the Swells..

Here ya go

"3. Wave speed. The speed of a swell or a wave train can be calculated by multiplying the swell period times 1.5. For example, a swell or a wave train with a period of 20 seconds will be traveling at 30 knots in deep water. (Knots are nautical miles per hour. One knot equals 1.2 mph on land.) A swell with a period of 10 seconds will travel at 15 knots. The individual waves actually move twice as fast as the wave train or the swell, and a single wave's speed can be calculated by multiplying the swell period times three. So individual waves with a period of 20 seconds travel at 60 knots in deep water. Again, think of the wave train like a rotating conveyor belt that is also moving forward."

I knew surfers would have the answer
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Old 28-07-2012, 22:10   #40
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Re: Sailing between the Swells..

Just looked at the briefing package for the Pacific. Swells predicted at 7 seconds. So swell speed is about 11 knots. With you going downwind at 7 knots, you have a speed of 4 seconds, so a swell hits you about every 14 seconds or so right? More importantly, you could probably surf down that swell and maybe stay in between them for quite some time. Am I correct in this thinking?
But going upswell, you will encounter a swell every 4 seconds or so. Nasty stuff. Any recommendations for that?
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Old 29-07-2012, 01:07   #41
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Just looked at the briefing package for the Pacific. Swells predicted at 7 seconds. So swell speed is about 11 knots.
7 * 3 = 21 knots

Hard to believe, but true for swells.

The 11 knot speed is how fast the system of waves moves across the sea. Important for predicting conditions. The lead waves of a system don't last long. Observe the bow wave of your dinghy - lives less than a second.
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Old 29-07-2012, 02:36   #42
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One thing to note is that it takes alot of fetch - read hundreds of miles - for locally wind generated waves to reach their full speed. The closer to point of generation the steeper they are, slower, and tighter period.

For example when sailing in the south atlantic it is very common to have swells from the south with periods of 20 sec or more that are over 3 meters. They are hardly noticable or a problem, until locally developed swells from the east or west buildup on top
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Old 29-07-2012, 02:41   #43
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Quote:
Originally Posted by s/v Beth
Just looked at the briefing package for the Pacific. Swells predicted at 7 seconds. So swell speed is about 11 knots. With you going downwind at 7 knots, you have a speed of 4 seconds, so a swell hits you about every 14 seconds or so right? More importantly, you could probably surf down that swell and maybe stay in between them for quite some time. Am I correct in this thinking?
But going upswell, you will encounter a swell every 4 seconds or so. Nasty stuff. Any recommendations for that?
The tighter the period the steeper the swell, generally. Imagine a sinewave, compress or enlongate it and the angles change. Not a perfect analogy but close enough. When in deep water you can generally only surf very locally generated waves.
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Old 05-08-2012, 03:53   #44
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Re: Sailing between the Swells..

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Huge swells of the 20 to 40 foot variety will pass the boat without any effect........

Wind driven or breaking waves of 20 feet are altogether different.
Aha... I had never actually clicked that there was a difference between the two... I always thought of them as just 'big waves' and 'little waves'.

But aren't the big swells also wind-driven? I mean, in the below case, I assume the big swells were 'old' waves from the storm up the Gulf, and the smaller ones up top were the fresh waves being freshly created by the storm. I'm not even sure if what I'm saying makes sense.... does it?

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...we were sailing back into wind from Bahrain when we hit a storm. 30' swells (up between the spreaders), and you never knew what you would hit at the top. Sometimes you would slide straight down the other side, sometimes there would be a mixed up chop that would bash right back at ya.
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Old 05-08-2012, 04:02   #45
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Big swells were wind driven at some earlier time. Probably by winds of 60 knots or more. And at that time probably steep and uncomfortable.
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